The bitterness of beer is due to the presence of iso-alpha acids. In traditional brewing, they are formed during the boiling of the hops in the kettle. This produces unreduced iso-alpha acids (UIA) which are unstable in light, producing off-flavors. To overcome this deficiency, hydrogenation of the unreduced iso-alpha acids (UIA) to form dihydro (DHIA), tetrahydro (THIA), and hexahydro (HHIA) iso-alpha acids has been commercialized, and these three different kinds of reduced molecules are now routinely used in the brewing art.
While all of the reduced kinds (forms) provide stability to light, the art has now recognized that the tetrahydro and hexahydro kinds increase the foam, cling, and mouthfeel as compared with unreduced or dihydro iso-alpha acids. In some instances, they are used for these effects alone, i.e., even though light stability is not required.
While all of these hop bitter acids can be added to the kettle in the form of a paste or solid, the preferred point of addition is after fermentation. This improves the control of the flavor and improves the efficiency with which the hop herb is converted into bitter acids. Formulations for post-fermentation addition of iso-alpha acids are described in three patents, all of which permit dilution in water to form 1% to 2% solutions of the bitter acids prior to addition to the beer. The formulations employed also depend upon the pH adjustment of the liquid iso-alpha acid composition to achieve a stable, single-phase, concentrated, liquid solution.
In the art, the term isohumulone has been used generically to describe the collection of analogues, which differ in a side chain. At the present time, the term iso-alpha acid is preferably used to describe this mixture. For the purposes of this specification, the term iso-alpha acid will be used and include all the analogues present in the parent hop. Furthermore, di-,tetra-, and hexa-hydro denote the addition of two, four, or six hydrogens to the unreduced molecule without changing the side chain. Accordingly, four distinct molecular kinds of iso-alpha acids are used commercially. Each kind has its specific attributes and behavior.
1. Mitchell, U.S. Pat. Nos. 3,973,092 and 3,973,052, describes a concentrated soft extract, consisting of potassium isohumulate (UIA), and containing 15% to 25% water. In addition, he claims a mixture consisting of 40% to 60% isohumulate in the former patent, and a mixture containing 16% to 72% water (84% to 28% isohumulate) in the latter patent. While not describing the preferred concentration, the commercial practice is now to provide a 30% solution of iso-alpha acids to the brewer. This solution is stable, does not separate into two phases on standing, and can be added to water to make a 1% to 2% solution of iso-alpha acids, which is also stable. This can then be dosed into the fermented beer.
2. Westermann, U.S. Pat. No. 3,798,332, describes stable solutions of dihydro iso-alpha acids (DHIA), and finds that a single-phase aqueous solution can be obtained at concentrations greater than 30%, and optimally between about 35% and 40%. This product, which is light stable, is commercially provided as a 35% solution in water.
It should be noted that there is a critical lower limit to the concentration of the iso-alpha acids in both the Mitchell and Westermann formulations, it being 28% for the former, and 30% for the latter.
3. Todd, U.S. Pat. No. 3,486,906, showed that when propylene glycol or glycerine were used as carriers, a stable single-phase solution could be made containing between about 5% and 30% or more by weight of iso-alpha acids, which include all the kinds and forms of the bitter acids mentioned above, provided the solutions contain less than 25% water. These solutions are available commercially at 20% to 30% iso-alpha acid concentration.
However, the tetrahydro and hexahydro kinds of alpha-acids do not behave like the unreduced or dihydro kinds in water. They cannot be made into stable single-phase solutions at the commercially desirable concentrations of 20% to 35%. Tetrahydro can be made into a stable single-phase solution in water at a maximum concentration of 10% to 11%, and is provided commercially as a 10% solution. Hexahydro is not soluble in water above about 5%, and is not available except as a solution in propylene glycol. The presently-preferred concentrations of 20% to 30% of these two desirable reduced kinds must therefore be formulated according to Todd, using propylene glycol or glycerine.
In summary, the art shows that the concentration for stable aqueous solutions of unreduced and dihydro iso-alpha acids must be 28% to 30% minimum, for tetrahydro it is 10% maximum, and there is presently no commercially viable aqueous solution of hexahydro.